Formation of corrosion-resistant metallic coatings by so-called flame-spraying techniques



3,025,182 FORMATION OF CORROSilflN-RESKSTANT METAL- LlC CQATINGS BY Sift-CALLED FLAME-SPRAY- ING TECHNIQUES Nils Gustav Schrewelius, Hallstahammar, Sweden, assignor to Aktiebolaget Kanthal, Hallstahammar, Sweden No Drawing. Filed Mar. 3, 1958, Ser. No. 737,465 Claims priority, application Sweden Mar. 5, 1957 8 (Zlaims. (Cl. 117105) The present invention relates to an improved method of producing corrosion-resistant, particularly oxidationresistant metallic coatings.

Such metallic coatings as are obtained by a so-called flame-spraying (spray-metallization) process will generally contain pores, generally to an extent corresponding to to 30% by volume of the material. It is generally desirable that the coatings should afford a satisfactory corrosion-resistance, particularly oxidation-resistance qualities, and with this purpose in view it has been proposed to use as a basis one or more of the metals Al, Cr and Si which exhibit the common feature that their oxides are non-reducible or very diflicultly reducible by gaseous hydrogen. Metallic particles containing A1, Cr or Si, therefore, are readily coated by a difiicultly reducible oxide skin which, however, will obstruct the metallic sintering together of the particles to a non-porous product, which will prove to be a considerable draw-back in a flame spraying procedure.

The present invention has for its object to provide a method of avoiding said drawback and to produce corrosion resistant flame-sprayed coatings in which there are metallic contacts between the particles so that a strong metallic skeleton is provided in the coating and a reduced porosity obtained.

The method of the invention is illustrated in the following fiow diagram, interpretation of which will be found in the disclosure below:

Solid boron component with Metal component more than 0.2%

boron Wire, rod Wire, rod or powder or powder 60-93% b.w, 2-4072, b.w.

Flam spraying mixture of metal component and boron component Coated base material According to the present invention it has been found,

[i seas 18 2' HQ Patented Mar. 1 1952 elements constituting at least 1% of the Weight of the component and possibly a balance constituted by one or more other metals, such as Ti, Zr, V, Nb, Ta, Mo, W, Mn, Fe, Co, and Ni, as well as by the mutual alloys and intermetallic compounds thereof and, secondly, of a boron component comprising boron and/ or one or more borides, such as the borides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni and Si, the boron content being at least 0.2% of the weight of the boron component and, in any case, the total content of boron being within the range of 0.08 to 10% by weight of the mixture intended for flame spraying and said metallic component constituting 60 to 98% by weight and said boron component 40 to 2% by weight ofsaid mixture.

In order to assure a sufficiently high liquids temperature the elements Al, Cr and Si should be so selected that, if only one of them occur, the content of Al should not exceed 25% of the metallic component, that of Cr not and that of Si not 50% of the metallic component.

Particularly good results have been obtained when a considerable portion of the metallic component is constituted by molybdenum silicide, particularly MoSi or by a molybdenum-silicon-aluminium-alloy. More particularly, the metallic component has the following composition: 15 to 40% Si, 0 to 30% Al and 30 to Mo.

It should also be noted that most metals which have been in contact with air have a more or less pronounced oxide film. Particularly, when it is a question about powder grains of microsize the oxygen content of the oxide film may reach noticeable percentages of the weight of the metallic component. This oxygen content which, of course, may vary from one case to another is, however, not included in the percentages mentioned in the present application.

The boron component may consist of boron only or of boron together with one or more borides or of borides only.

The mixing together of the two components according to the invention can be effected in any one of several different ways. According to one way of carrying the invention into effect, the two powdered components can be intermixed physically, after which the powder-mixture is sprayed according to methods well-known by those skilled in the art, such as by utilizing the powder-mixture as such by first admixing therewith a plastic binder and then forming the mass into a strand, which is fed onto the place for the flame spraying. According to another way of carrying the invention into effect, it is possible to use simultaneously two spraying devices, viz. one for each of the two components. According to a further way of carrying out the invention, one of the two components-preferably the metallic componentcan be formed into a core which has applied onto it a surface coating or layer of the boron component. This is preferably effected by forming a wire or rod of uniform thickness while coating a strand made by the material of the one component with the material of the other component, for instance by a preparatory flame-spraying operation after which said wire or rod is used as a unitary structure for providing material for the intended final flamespraying operation.

Further possible methods of realizing a physical mixture of the two components are also conceivable, however. In any case, in the flame-spraying process solid particles of two different components of material will be thrown simultaneously on to the base material to be coated.

Without being confined by my theory, it may be assumed hypothetically that the particles of the boron component applied are capable of reacting with the particles of the metallic component in such a manner that boron is caused to flux and/or reduce the oxide skins formed on the surface of these particles of the metallic component. In flame-spraying processes the material particles will be fused into a fiuid state, and owing to the creation of an intense rise of temperature for a short time far above the melting point of the particles, reactions will be enabled which would be attained only with difficulty, if at all, at lower temperatures such as would obtain in a powder-metallurgical sintering process, for example, in which the temperatures are generally only about 0.7 to 0.8 of melting temperature. For this reason it is important that the composition of the metallic and boron components is so selected that the temperature of the liquidus state will be at least 1300 C. Also, the high temperature associated with the process according to the present invention will contribute to the dissipation of substantial amounts of the boric acid formed, which will volatilize, and the continuous, sintering-inhibiting oxide layers about the metallic particles will fuse and combine into a much smaller number of oxide grains in the flame-sprayed coating obtained, which facilitates the sintering process. A great technical advantage resides in the fact that boron introduced will largely volatilize and thus disappear from the flame-sprayed coating, since in certain cases boron may have an adverse influence on the corrosion-resistance of the coatings obtained. Since according to the invention, boron will always be introduced in the form of individual boroncontaining particles, a superficial reaction will primarily be involved, and thus the other particles which do not contain boron will dissolve boron to a very slight extent only. Thus, in this respect the invention distinguishes from those prior-art methods according to which a metallic powder to be used in a powder-metallurgical production process, for example, contains dissolved boron which upon sintering the material exerts a dissolving or fluxing effect. It is difficult, as a rule, to cause volatilization of such boron to an extent comparable to that of the present invention, since in sintering processes it will not be possible to create such high temperatures of short duration as will be produced in accordance with the present invention.

Thus, the corrosion-resistant metallic materials capable of being used in the metallic component according to the present invention are to contain at least 1% Al, Cr and/ or Si. By way of example, metallic components may be mentioned which to their major portion consists of an oxidation-proof ferro-alloy having chromium, aluminium and cobalt as constituents and containing -40% Cr, l-9% Al, /2-8% Co, 0.0l0.5% C, the balance being Fe. The ferro-alloys may also be of the so-called stainless type having, for instance, 18% Cr and 8% Ni. The metallic components may also to a major portion consist of a nickel-aluminium alloy. Suitable boron components are, for instance, ferroboron containing 11% B, zirconium-diboride ZrB and dichromium-boride Cr B. Also possible to use are pure boron or boron-containing alloys, preferably based on nickel. Examples of such alloys are: 70 to 80% Ni, 10 to Cr, 3 to 6% Si, 3 to 6% Fe and 2 to 3% B, or: 93% Ni, 4% Si and 2.3% B. The first-mentioned alloys are known to be suitable materials for flame-spraying purposes and are characterized by a high resistance to oxidation, good wetting characteristics, and a wide melting range (1000 to 1100 C.), whereby they are suitable for a subsequent densification by fusing the same by means of a welding torch. However, without being subjected to such subsequent treatment, they will exhibit a substantial degree of porosity which makes them ineffective as a protection against corrosion.

The thickness of the coatings may be varied within a wide range, suitably from 10 micra and upwards. Where the coeflicients of thermal expansion of the coating and the base material are widely different, the coating should be thin, or it is possible to use intervening bonding layers having intermediate coefficients of thermal expansion, which layers may be sprayed in accordance with the present invention or may be applied in any suitable other way.

Coatings produced according to the invention may be applied onto metallic materials, such as cast iron, stainless steels, Monel metal, nickel, copper, as well as onto ceramic materials. They may find numerous industrial applications, for instance, in the chemical industry, as construction materials and as electric resistance materials, etc.

It is possible also to spray the coating material on to a destructible material, such as, for example, a closemeshed wire gauze which is subsequently removed by a corrosion process. In this way articles of complicated shapes may be produced.

Various methods of carrying the invention into effect will appear from the following examples:

Example 1 A powder of an alloy is produced by atomization under water vapour. The powder has the following composition: 4.5% A1, 3% A1 0 23% Cr, 0.8% Co, 0.03% C, the balance to being iron, and the aluminum oxide being present in the form of a surface layer on the particles. From this powder the fraction from to 200 (U.S.) mesh is screened off; 97% by weight of this powder constituting the metallic component are mixed with 3% by weight of ferroboron constituting the boron component and having a grain size of less than 325 (U.S.) mesh and containing 11.5% boron. The powder mixture is pasted with 15% polyethylene, is then extruded into a flexible wire and is flame-sprayed in a gaseous mixture of acetylene and oxygen. A coating will be obtained containing 0.01% boron and having a porosity of 2%.

Example 2 A drawn and rectified 2.98 mm. wire representing the metallic component and consisting of 23% Cr, 5% Al, 1% Co, balance Fe, is pickled and has applied to it by a preparatory flame-spraying a uniform 0.14 mm. coating representing the boron component and having the following composition: 12% Cr, 4% Fe, 4% Si, 2.4% B, 0.5% C, balance Ni. The spray-coated wire, which contains 15% boron-containing and 85% aluminum-containing alloy, has a diameter of 3.25 mm. and is used in its turn for the final flame-spraying purposes. The coating thereby obtained contains 0.2% B and shows on a microscopic examination of an etched and polished microstructure sample, that the flattened metal particles having the approximate size of 25 micra are separated by austenitic streaks. Oxide particles are present, but they are relatively few in number and seem not to prevent the wetting effect between the metal grains. No porosity whatever could be detected microscopically.

Example 3 Produced according to powder-metallurgical techniques is a straight rod representing the metallic component and having a uniform thickness of 3.2 mm. and containing 13.5% Al, the balance being Ni, corresponding to the intermetallic composition Ni Al. Additionally used is a powder mixture representing the boron component and containing 4% Si, 2.3% B, the balance being Ni, the grain size ranging from 120 to 200 (U.S.) mesh. The flame-spraying operation is carried out with the simultaneous use of two spray-guns, one of these guns being fed with the uniform Ni Al-rod and the other being supplied with said boron component powder. The spraying process is so arranged as to cause the spray bundles or jets from the two guns to intersect or coincide simultaneously at the base surface to be coated with the metallic layer, the consumption per unit time of NigAl then being 10 times that of said powder. The coating obtained in this way has the following composition: 11% A1, 0.5% Si,

0.07% B, balance Ni, and has a porosity of 4%, the pores, however, being closed throughout.

Example 4 Produced according to powder-metallurgical techniques is an alloy containing 30% Al, 25% Si and 45% M0. The alloy is ground into a grain size of less than 10 micra. From this powder representing the metallic component 95 parts by weight are admixed with parts by weight of CrB representing the boron component and produced by melt-metallurgical techniques and also ground into a grain size below micra. This powder mixture is flame-sprayed and yields a coating of the following composition: 27% Al, 23% Si, 45% Mo, 4% Cr and 1% B.

Example 5 In a ball mill a mixture of 5 parts by weight of amorphous boron representing the boron component and 95 parts by weight of an alloy representing the metallic component and containing 45% Si and 55% M0 is ground to a grain size below 8 micra. The powder mixture is admixed with polyethylene, as in Example 1, and is flamesprayed in an atmosphere of acetylene and oxygen gas. The coating obtained, which exhibits closed pores exclusively, has the following composition: 35% Si, 0.5% B, the balance being Mo.

Example 6 A wire of a stainless ferro alloy of the type 18/8 and constituting the metallic component, has sprayed onto the same a boron-containing alloy in a manner exactly analogous to that described in Example 2. The flame-sprayed coating obtaining, being austenitic throughout, will be found to contain a few oxide particles, but no open porosity whatever.

What I claim is:

l. The method of producing corrosion-resistant metallic coatings characterized by flame-spraying upon a surface to be coated a mixture consisting essentially of two diiferent components at the same time, said first component being a metal component in the form of a first body and consisting essentially of to 40% silicon, 0 to 30% aluminum and 30 to 85% molybdenum, said second component being a solid boron component in the form of a second body distinct from said metal component and having fluxing properties and being capable of substantially preventing oxidation of said ingredients of the first component during flame-spraying, the boron content of said boron component being at least 0.2% by weight of the boron component and within the range of 0.08 to 10% by weight of the mixture intended for flame spraying, said metallic component constituting 60 to 98% by Weight and the boron component 40 to 2% by weight of said mixture.

2. A method according to claim 1 wherein the metallic component consists essentially of molybdenum silicide.

3.A method according to claim 2 wherein said two components are mixed with each other in powder form and are then subjected to flame spraying.

4. A method according to claim 1 wherein each of said components is fed from a separate spraying device arranged in such a manner as to cause jets projecting from the two spraying devices to coincide simultaneously on the base material surface to be coated.

5. The method of. producing corrosion-resistant metallic coatings characterized by flame-spraying upon a surface to be coated a mixture consisting essentially of two diiferent components at the same time, said first component being a metal component in the form of a first body and consisting essentially of a ferro-alloy having 15-40% Cr, 1-9% Al, /z-8% Co, 0.010.5% C, balance Fe, said second component in the form of a second body distinct from said metal component and being a solid boron component having fluxing properties and being capable of substantially preventing oxidation of said ingredients of the first component during flame-spraying, the boron content of said boron component being at least 0.2% by weight of the boron component and within the range of 0.08 to 10% by weight of the mixture intended for flame spraying, said metal component constituting 60 to 98% by weight and the boron component 40 to 2% by weight of said mixture.

6. A method according to claim 5 wherein said two components are mixed with each other in powder form and are then subjected to flame spraying.

7. A method according to claim 5 wherein each of said components is fed from a separate spraying device arranged in such a manner as to cause jets projecting from the two spraying devices to coincide simultaneously on the base material surface to be coated.

8. The method of producing corrosion resistant metallic coatings characterized by flame-spraying upon a surface to be coated a mixture consisting essentially of two different components at the same time, said first component being a ferro-alloy having 15-40% Cr, 19% Al, /z--8% Co, 0.01-0.5% C, balance Fe, said first component being in the form of a core of wire or rod of uniform thickness, said second component being a solid boron component having fluxing properties, the boron content of said boron component being at least 0.2% by weight of the boron component and within the range of 0.08 to 10% by weight of the mixture intended for flame spraying, said boron component being in the form of a coating of uniform thickness on the surface of said core, said ferro-alloy component constituting 60 to 98% by weight and said boron component 40 to 2% by weight of said mixture.

References Cited in the file of this patent UNITED STATES PATENTS 2,203,180 Brix Oct. 31, 1916 1,675,798 Franks July 3, 1928 2,152,637 Golyer Apr. 4, 1939 2,262,072 Vaughn Nov. 11, 1941 2,570,649 Davidoff Oct. 9, 1951 2,661,285 Gorschalki Dec. 1, 1953 2,864,696 Foreman Dec. 16, 1958 2,868,667 Bowles Jan. 13, 1959 2,874,065 Herz et al. Feb. 17, 1959 2,930,106 Wrotnowski Mar. 29, 1960 FOREIGN PATENTS 468,753 Great Britain July 8, 1937 543,773 Great Britain Mar. 12, 1942 

1. THE METHOD OF PRODUCING CORROSION-RESISTANT METALLIC COATINGS CHARACTERIZED BY FLAME-SPRAYING UPON A SURFACE TO BE COATED A MIXTURE CONSISTING ESSENTIALLY OF TWO DIFFERENT COMPONENTS AT THE SAME TIME, SAID FIRST COMPONENT BEING A METAL COMPONENT IN THE FORM OF A FIRST BODY AND CONSISTING ESSENTIALLY OF 15 TO 40% SILICON, 0 TO 30% ALUMINUM AND 30 TO 85% MOLYBDENUM, SAID SECOND COMPONENT BEING A SOLID BORON COMPONENT IN THE FORM OF A SECOND BODY DISTINCT FROM SAID METAL COMPONENT AND HAVING FLUXING PROPERTIES AND BEING CAPABLE OF SUBSTANTIALLY PREVENTING OXIDATION OF SAID INGREDIENTS OF THE FIRST COMPONENT DURING FLAME-SPRAYING, THE BORON CONTENT OF SAID BORON COMPONENT BEING AT LEAST 0.2% BY WEIGHT OF THE BORON COMPONENT AND WITHIN THE RANGE OF 0.08 TO 10% BY WEIGHT OF THE MIXTURE INTENDED FOR FLAME SPRAYING, SAID METALLIC COMPONENT CONSTITUTING 60 TO 98% BY WEIGHT AND THE BORON COMPONENT 40 TO 2% BY WEIGHT OF SAID MIXTURE. 